Optical resonators for lasers have typically consisted of spaced reflecting surfaces which reflect light rays repetitively through a gain region where laser pumping takes place. Energy is extracted from the resonator around the outside edge of one of the mirrors, or by providing an opening in one of the mirrors, or by partial transmission through one of the mirror surfaces so that a portion of the light rays are transmitted through or around the mirror while the remaining portion is reflected back through the gain region.
In some types of lasers it is desirable to provide a resonator in which the light rays pass through an annular cylindrical shaped gain region. This has been accomplished in the past by means of a conical mirror unit. The conical mirror unit, sometimes referred to as a W-axicon or reflaxicon, consists of a central 90.degree. conical mirror surrounded by an outer conical mirror having a 90.degree. conical surface. Light directed in a compact beam along the axis of the central conical mirror is reflected radially outwardly at 90.degree. to the axis of the cone where it impinges on the conical reflecting surface of the surrounding outer conical mirror. The conical surface of the outer conical mirror in turn directs the light rays in a direction parallel to the axis of the cone within an annular cylindrical volume concentric with the axis of the cones. A rear conical ring mirror may be used at the opposite end of the annular gain media to reflect the light radially across the diameter of the cylinder to the other side of the ring where it reflects back parallel to the axis of the cone and goes back to the W-axicon.
In order to use a ring laser with an annular cylindrical gain media it is necessary for the W-axicon (or reflaxicon) to provide for separate input and output beams. The standard method is to use biconic W-axicons (or reflaxicons) with the cone angles such that the chief ray in the annular beam is not parallel to the cone axis and with some optical power (curvature) on the conical elements. The rear cone ring mirror also has optical power and small deviations from 90.degree.. As a result, the annular input beam from one element of the biconic W-axicon diverges until it strikes the rear cone ring mirror which converts it to a converging annular beam and returns it to the output element (as opposed to the same input element) of the biconic W-axicon. As with every ring laser, there is a reverse mode which enters the annular gain media through the "output" element of the biconic waxicon and leaves through the "input" element. It is important to suppress this parasitic reverse mode. Unfortunately any "cross talk" or transfer of energy between the forward and reverse modes enhances and aggravates the reverse mode parasite. Diffraction of the beams causes some cross talk or energy transfer by spreading the annular beams so that portions of the beam intended for one of the biconic elements strikes the other closely spaced biconic element. This situation becomes more serious as the annular gain media is made thinner and longer. It limits the size and shape of the gain media in these devices.